The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor through which halogenated aromatic hydrocarbons, such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), cause altered gene expression and toxicity. The AHR belongs to the basic-helix-loop-helix/Per-ARNT-Sim (bHLH-PAS) family of transcriptional regulatory proteins, whose members play key roles in development, circadian rhythmicity, and environmental homeostasis. However, the normal cellular function of the AHR is not yet known, despite extensive studies of the AHR in mammals, including AHR-null mice. Fish and non-mammalian vertebrates are being used increasingly as model systems in toxicology and carcinogenesis research, in developmental biology, and as sentinels of environmental quality. Proper interpretation of results obtained in such models requires an understanding of the similarities and differences in molecular mechanisms between humans and fish. In the preceding grant period, we identified an AHR homolog (AHR1) as well as a novel AHR-related gene (AHR2) in bony and cartilaginous fish. Fish AHR1 and AHR2 both exhibit specific, high-affinity binding to [3H]TCDD. Phylogenetic analyses show that an AHR gene duplication occurred early in vertebrate evolution, suggesting that two AHR genes could exist in other vertebrates, including humans (PNAS (1997) 94:13743). To fully understand TCDD effects in humans, it is critical to establish whether humans possess a second AHR and, if so, whether it functions as a ligand-activated transcription factor like AHR1. If an AHR2 is not present in humans, it is important to determine the functional relationships between fish AHR1/AHRs and the human AHR in order to assess the value of fish as models in environmental health research. The overall objectives of the proposed research is to assess the similarities and differences in components and mechanisms of AHR-dependent signal transduction between humans and fish used as models in toxicology. We will determine the diversity and function of AHR genes in fish species commonly used as aquatic models, test the hypothesis that a second AHR-like gene exists in humans, and compare the function of fish and human AHRs.